System for removing sodium chloride contaminants from a magnesium base pulping process

ABSTRACT

A chemical absorption system for removing hydrogen chloride from gases resulting from the combustion of magnesium base pulp residual liquor containing sodium chloride.

United States Patent 91 Flais et a1.

[ Oct. 29, 1974 Inventors: Louis R. Flais; Robert A. Mcllroy, both of Alliance, Ohio Assignee: The Babcock & Wilcox Company,

New York, NY.

Filed: Oct. 30, 1972 Appl. No.: 302,375

US. Cl 162/36, 55/71, 55/DIG. 7, l62/DIG. 8

Int. Cl. D210 11/02 Field of Search 162/30, 36; 423/488; 55/DIG. 7, 71

[56] References Cited UNITED STATES PATENTS 2,127,571 8/1938 Pardee, Jr 55/71 2,978,378 4/1961 Hutt et al 162/30 3,046,182 7/1962 Tomlinson et a1. 162/36 3,092,535 6/1963 Tomlinson 162/36 3,269,095 8/1966 Brannland 55/71 Primary Examiner-S. Leon Bashore Assistant Examiner-William F. Smith [5 7] ABSTRACT A chemical absorption system for removing hydrogen chloride from gases resulting from the combustion of magnesium base pulp residual liquor containing sodium chloride.

2 Claims, 2 Drawing Figures 13 WATER DISCARD minimum 29 m4 3;e44;a79

sum an: 2

45 43 61 46 44 MAKE-UP L WATER v 52 WASH WATER 4 r54 TO 502 RECOVERY APPARATUS g 7 SYSTEM FOR REMOVING SODIUM CHLORIDE CONTAMINANTS FROM A MAGNESIUM BASE PULPING PROCESS The present invention relates to the recovery of heat and chemicals from the residual pulp liquor developed during the digestion of cellulosic material in the magnesium base cooking liquor, and more particularly to the selective removal of a deleterious chemical from the recovery cycle in the regeneration of magnesium base cooking liquor.

The use of magnesium base cooking liquor in the pulping of wood chips is well known, as is the recovery process for reclaiming chemicals and heat from the pulp residual liquor with the reclaimed chemicals recycled in the pulp cooking cycle. This system is shown, for example, in US. Pat. Nos. 3,046,182 and 3,092,535.

In some installations of the magnesium base pulping and chemical recovery type, the wood is water borne in transit to the pulp mill and sometimes stored in salt water, and it has been found the wood chips contain sodium chloride. During subsequent processing of the residual liquor, various forms of chlorine compounds are formed which have had an excessive corrosive influence on the chemical recovery equipment. The chlorine has been found to be predominently present in the gases of combustion leaving the incinerating furnace, in the form of gaseous HCl (hydrogen chloride) with a minor amount of free chlorine. The sodium introduced into the system as sodium chloride combines with sulphur in the incinerating furnace to form particulate sodium sulphate, which is air borne with the particulate magnesium oxide in the flue gases leaving the furnace.

In accordance with our invention, the flue gases discharging from the incinerating furnace are treated in dust separators to remove magnesium oxide and sodium sulphate particles and then passes through a gasliquid contact device to absorb the HCl as magnesium chloride without appreciable absorption of the S present in the flue gases. The flue gases, substantially freed of contaminating chlorine compounds, are then passed through 80-; absorbing apparatus for removal of the S0 and discharge of the flue gases to the atmosphere. The absorbed chlorine containing liquid is separately treated to remove chloride which is discharged from the system and the remaining liquid recycled under controlled conditions to the HCl gas-liquid contact device.

Of the drawings:

FIG. 1 is a line diagram of a magnesium base pulping and chemical recovery system incorporating a chlorine removal device arranged in accordance with the present invention; and

FIG. 2 is a schematic showing, on an enlarged scale, of an improved chlorine removal device shown in FIG. 1.

In the usual magnesium base pulping and chemical recovery system as shown in FIG. 1, wood chips are contacted with cooking liquid for a selected period at desired temperature and pressure in a digester 10, with the pulp delivered to a blow tank 11 and then passed through a washer l2. The washed pulp is delivered to a storage tank 13 for subsequent treatment, while the residual pulp liquor is passed through a pipe 14 to multiple-effect evaporators 15 for suitable concentration.

Ordinarily. the residual liquor is finally concentrated in a direct contact evaporator, symbolically shown at 16, before delivery to a burner or burners 17 for introduction into a furnace 18. The residual liquor is burned in the furnace 18 with the heat partially used to generate steam in an associated boiler 20, and the partially cooled gases further cooled in evaporating moisture from and further concentrating the residual liquor in the evaporator 16.

It is well known in the art, that the gaseous combustion products leaving the furnace contain sulphur oxides, and the gases also entrain particulate magnesium oxide. The particulate matter including sodium sulphate is separated from the carrier gases in a separator 21 with the separated solids passes through a discharge pipe 29 to a purifying system to remove impurities and a slaking tank 19 with the resulting magnesium oxide containing slurry subsequently utilized for S0 absorption in a system hereinafter described.

In the embodiment shown, the separator 21 is illustrated as being of the mechanical type, and is typical of operating installations utilizing magnesium base cooking liquor and having a chemical recovery system. Such a separator has a particulate removal efficiency of about percent, and the hereinafter described chlorine removal system is based on the use of mechanical dust separators. Thus, the invention may be added to the chemical recovery system described without major changes in existing equipment.

The flue gases from the separator 21 and direct contact evaporator 16 pass through a duct 22, a scrubber and chlorine disposal device 23, and thence through a duct 24 to a direct contact cooling tower 25. The device 23 is shown in detail in FIG. 2 and hereinafter described. From the tower 25 the cooled gases pass through a duct 26 and an S0 absorption apparatus, symbolically shown at 27, before discharge to the at mosphere through duct 28. Such an S0 absorption system is disclosed in US. Pat. No. 3,273,961. In this system the gases are first cooled to a desirable temperature in a tower 25 and then the gases containing the S0 are contacted by a spray of magnesium containing liquid. In the tower 25 the cooling liquid is recirculated from the bottom of the tower through a pipe 30, cooled by an indirect heat exchanger while in transit. and sprayed into the top of the tower. Make up water is added through pipe 31, and a portion of the discharge from the tower 25 is passed to the S0 absorption apparatus 27 through pipe 32. In the absorption apparatus 27 MgO slurry is added through pipe 33 from tank 19 and passed through pipe 34 and pump 35 with other liquid to absorb the S0 from the flue gases. As disclosed in the referred to Pat. No. 3,273,961, the absorption apparatus 27 usually includes two or more absorption devices, such as venturi scrubbers or the like, arranged in series with liquid containing the absorbed S0 passed through a pipe 36 to a fortification tower 37 where cooking liquor is prepared for use in the degester 10. The cooking liquor is passed, as required, from the fortification tower 37 through pipe 38 to the digester 10.

The chlorine removal apparatus 23 is shown in FIG. 1 to indicate its position in the overall chemical recovery system and is shown in detail in FIG. 2. Referring to FIG. 2, the flue gas entering the chlorine removal apparatus 23 through the duct 22 contains some MgO particles (due to the use of a mechanical separator), gaseous S and gaseous HCl as well as the usual products of combustion such as CO and N Sometimes, depending upon gaseous temperature conditions, free chlorine may also be present, but since only trace quantities are involved they may be ignored. The gases leaving the duct 22 enter a venturi where they are contacted in cocurrent relationship by a liquid spray injected through a nozzle 41. Substantially all of the MgO present in the flue gases will be removed in the gasliquid contact device or venturi 40 and will combine with some of the S0 to form a solution of magnesium bisulphite and will also combine with the chlorine to form a solution of magnesium chloride. In the usual chemical recovery system described the quantity of MgO available in the venturi 40 will concert a substantial amount of the HCl to MgCl and the excess MgO will combine with the S0 available to form g( 3)2' After leaving the venturi 40 the gases turn in an enlarged sump 42 where centrifugal forces and the enlarged cross-sectional flow area cause a large percentage of the entrained solids and liquid droplets to deposit in the bottom of the sump 42 and the gases to pass upwardly into the duct 24. The accumulated material in the bottom of the sump is withdrawn through pipe 43 and pump 44 at a controlled rate with the pump discharge passing through a pipe 45 which is provided with twovalve controlled branch pipes 46 and 47. The pipe 46 discharges into a pipe 48 which leads to the nozzles 41, and the pipe 47 discharges into a reactor tank 50.

.The reactor tank 50 receives a small portion of the liquid passing through the pump 44, and also receives a controlled flow of magnesium hydroxide Mg (OH) slurry through pipe 51. The tank 50 is provided with a mixing device 52 so that the solution of magnesium chloride Mg Cl and magnesium bisulphite Mg (H SO will be intimately mixed to precipitate crystals of magnesium sulphite Mg S0 according to the reaction:

lution of magnesium chloride Mg C1 the material may be dumped into the ocean without contamination. The major portion of the liquid withdrawn from the filter 54 will be passed through pipe to mingle with liquid from pipe 46 and make up water from pipe 61 for delivery to the nozzles 41. With the circuit described the magnesium chloride MgCl concentration in the recirculated liquid flow to the absorber 23 can be regulated to enhance the removal of chlorine from the flue gases.

As an alternative arrangement, the separator 21 of FIG. 1 could be of the high efficiency electrostatic type, instead of the mechanical separator described in connection with FIG. 2. Under these conditions of dust collection and efficiency of 98 to 98.4 percent would leave sufficient magnesium oxide entrained with the gases entering a scrubber. such as shown at 40 in FIG. 2, to form magnesium chloride in the liquid withdrawn through pump 44. Little, if any, magnesium bisulphite would be formed by the gas-liquid contact under such conditions. Thus, the liquid withdrawn through valved pipe 47 could be directed to discard without appreciable loss of either sulphur or magnesium. The quantity withdrawn through valved pipe 47 would be controlled for recirculation of liquid through pipe 48, with only make up water added through pipe 61, and thus to the spray nozzles 41. To enhance the HCl pickup and minimize S0 absorption, the liquid sprayed through nozzles 41 would be maintained at a pH of approximately 2, as controlled by the quantity of liquid discarded through pipe 47.

In the usual pulping and chemical recovery installation using magnesium base pulping liquor the economics of a high efficiency electrostatic separator would not be favorable due to high investment and operating costs. However, under some conditions as for example when make up chemical costs are unusually high, such an investment might be justified.

The cooling tower 25 shown and described in connection with FIG. 1 of drawing would not be necessary in any arrangement utilizing a high efficiency electrostatic separator since in the latter circumstance the volume of liquid recirculated to the nozzles 41 would be sufficient to cool the gases to an efficient S0 absorption condition in the scrubbers 27. This cooling tower may also be eliminated when it is desired to operate the S0 absorption system at temperature close to the adiabatic saturation temperature.

The chlorine removal system described will limit the chlorine content of the cooking liquor to a value of 300-500 p.p.m., which will be adequate to minimize corrosion in the stainless steel piping and storage equipment.

What is claimed is:

1. In a chemical recovery system wherein cellulosic materials containing sodium chloride are pulped in a magnesium base cooking liquor, and the residual liquor from the pulping process contains the sodium chloride from the cellulosic materials, the residual liquor is concentrated by evaporation and burned to produce hot flue gases containing sulphur dioxide and hydrogen chloride and entrained solids including sodium sulphate and magnesium oxide, the step of removing entrained solids from the gases, treating the solids to form a slurry of magnesium oxide and to discard the sodium sulphate, passing the flue gases through a sulphur dioxide absorbing zone in contact with the magnesium oxide containing slurry to form magnesium sulphur compounds, the improved step of washing the flue gases before they are passed to the sulphur dioxide absorbing zone to selectively remove the hydrogen chloride gases by absorption in a magnesium containing liquid, the magnesium containing liquid being formed by some of the entrained magnesium oxide solids passing with the flue gases to the hydrogen chloride washing step and combining with the washing liquid to form magnesium chloride and some of the gaseous sulphur oxides combines to form a solution of magnesium bisulphite in the liquid, recirculating a major portion of the liquid to the washing step to absorb hydrogen chloride,

discarding a portion of the liquid, adding make up water to control the concentration of magnesium chloride and magnesium bisulphite in the recirculated liquid, removing some of the liquid from the washing step to be reacted with magnesium hydroxide to precipitate magnesium sulphite, the reacted liquid is separated from the precipitate with the liquid recycled to the washing step, and the precipitate solid magnesium sulphite subsequently utilized in the sulphur dioxide absorbing step in forming cooking liquor.

2. In a chemical recovery system wherein cellulosic materials containing sodium chloride are pulped in a magnesium base cooking liquor, and the residual liquor from the pulping process contains the sodium chloride from the cellulosic materials, the residual liquor is concentrated by evaporation and burned to produce hot flue gases containing sulphur dioxide and hydrogen chloride and entrained solids including sodium sulphate and magnesium oxide, the step of removing over 97 percent of the entrained solids from the gases, treating the solids to form a slurry of magnesium oxide and to discard the sodium sulphate, passing the flue gases through a sulphur dioxide absorbing zone in contact with a magnesium oxide containing slurry to form magnesium sulphur compounds, the improved step of washing the flue gases before they are passed to the sulphur dioxide absorbing zone to remove the hydrogen chloride gases by absorption in a magnesium chloride liquid, recycling the liquid containing magnesium chloride and regulating the rate of discard of magnesium chloride from the washing step to control the pH of the recirculated liquid to improve hydrogen chloride absorption and minimize S0 absorption in the liquid washing step. 

1. IN A CHEMICAL RECOVERY SYSTEM WHEREIN CELLULOSIC MATERIALS CONTAINING SODIUM CHLORIDE ARE PULPED IN A MAGNESIUM BASE COOKING LIQUID, AND THE RESIDUAL LIQUOR FROM THE PULPING PROCESS CONTAINS THE SODIUM CHLORIDE FROM THE CELLULOSIC MATERIALS, THE RESIDUAL LIQUOR IS CONCENTRATED BY EVAPORATION AND BURNED TO PRODUCE HOT FLUE GASES CONTAINING SULPHUR DIOXIDE AND HYDROGEN CHLORIDE AND ENTRAINED SOLIDS INCLUDING SODIUM SULPHATE AND MAGNESIUM OXIDE, THE STEP OF REMOVING ENTRAINED SOLIDS FROM THE GASES, TREATING THE SOLIDS TO FORM A SLURRY OF MAGNESIUM OXIDE AND TO DISCARD THE SODIUM SULPHATE, PASSING THE FLUE GASES THROUGH A SULPHUR DIOXIDE ABSORBING ZONE IN CONTACT WITH THE MAGNESIUM OXIDE CONTAINING SLURRY TO FORM MAGNESIUM SULPHUR COMPOUNDS, THE IMPROVED STEP OF WASHING THE FLUE GASES BEFORE THEY ARE PASSED TO THE SULPHUR DIOXIDE ABSORBING ZONE TO SELECTIVELY REMOVE THE HYDROGEN CHLORIDE GASES BY ABSORPTION IN A MAGNESIUM CONTAINING LIQUID, THE MAGNESIUM CONTAINING LIQUID BEING FORMED BY SOME OF THE ENTRAINED MAGNESIUM OXIDE SOLIDS PASSING WITH THE FLUE GASES TO THE HYDROGEN CHLORIDE WASHING STEP AND COMBINING WITH THE WASHING LIQUID TO FORM MAGNESIUM CHLORIDE AND SOME OF THE GASEOUS SULPHUR OXIDES COMBINES TO FORM A SOLUTION OF MAGNESIUM BISULPHITE IN THE LIQUID, RECIRCULATING A MAJOR PORTION OF THE LIQUID TO THE WASHING STEP TO ABSORB HYDROGEN CHLORIDE, DISCARDING A PORTION OF THE LIQUID, ADDING MAKE UP WATER TO CONTROL THE CONCENTRATION OF MAGNESIUM CHLORIDE AND MAGNESIUM BISULPHITE IN THE RECIRCULATED LIQUID, REMOVING SOME OF THE LIQUID FROM THE WASHING STEP TO BE REACTED WITH MAGNESIUM HYDROXIDE TO PRECIPITATE MAGNESIUM SULPHITE, THE REACTED LIQUID IS SEPARATED FROM THE PRECIPITATE WITH THE LIQUID RECYCLED TO THE WASHING STEP, AND THE PRECIPITATE SOLID MAGNESIUM SULPHITE SUBSEQUENTLY UTILIZED IN THE SULPHUR DIOXIDE ABSORBING STEP IN FORMING COOKING LIQUOR.
 2. In a chemical recovery system wherein cellulosic materials containing sodium chloride are pulped in a magnesium base cooking liquor, and the residual liquor from the pulping process contains the sodium chloride from the cellulosic materials, the residual liquor is concentrated by evaporation and burned to produce hot flue gases containing sulphur dioxide and hydrogen chloride and entrained solids including sodium sulphate and magnesium oxide, the step of removing over 97 percent of the entrained solids from the gases, treating the solids to form a slurry of magnesium oxide and to discard the sodium sulphate, passing the flue gases through a sulphur dioxide absorbing zone in contact with a magnesium oxide containing slurry to form magnesium sulphur compounds, the improved step of washing the flue gases before they are passed to the sulphur dioxide absorbing zone to remove the hydrogen chloride gases by absorption in a magnesium chloride liquid, recycling the liquid containing magnesium chloride and regulating the rate Of discard of magnesium chloride from the washing step to control the pH of the recirculated liquid to improve hydrogen chloride absorption and minimize SO2 absorption in the liquid washing step. 